This invention relates to a method and apparatus for achieving high reduction continuous hot rolling of ferrous and non-ferrous products such as billets, bars, rods and the like in a compact series of roll passes.
In any rolling operation, the work rolls exert pressure on the product passing through the roll pass. This pressure is accompanied by frictional forces resulting from the difference in speed between the metal being rolled and the roll surfaces. The vertical components of roll pressure and friction act to reduce the height of the product. The horizontal components of roll pressure act opposite to the direction of rolling and tend to eject metal from the roll gap, whereas the horizontal components of frictional forces act in the direction of rolling in the zone of backward slip and tend to draw the product into the roll gap. In the following discussion, forces acting on the product in the direction of rolling will be considered as positive forces, and those acting on the product opposite to the direction of rolling will be considered as negative forces.
As a product leading end enters a roll pass, the algebraic sum of the horizontal force components of roll pressure and friction will undergo a continuous change from the time that the leading end initially contacts the rolls until it emerges from the roll gap. If this sum remains positive throughout this entry stage, the leading end will be gripped by the work rolls and drawn into and through the roll gap, and this will occur without assistance from any additional force. This condition will be referred to hereinafter as "spontaneous entry".
On the other hand, if the algebraic sum of horizontal force components achieves a negative value during the entry stage, then additional force must be exerted on the product in advance of the roll pass in order to achieve entry. This condition will be referred to hereinafter as "forced entry".
After the roll gap is filled and a condition of equilibrium has been reached, the sum of these horizontal force components will equal zero.
It has been established theoretically that spontaneous entry will occur if the bite angle .alpha. is kept within the range EQU 0&lt;.alpha..ltoreq..rho.
where .rho. is the angle of friction.
Conversely, a condition of forced entry will exist where EQU .alpha.&gt;.rho.
It has also been established that once a leading end has entered the roll pass and the roll gap is filled, free rolling will continue within the theoretical limits EQU 0&lt;.alpha..ltoreq.2.rho.
As herein employed, the term "free rolling" means rolling without using additional force to push or pull the product through the roll pass after the roll gap is filled. If the bite angle exceeds the theoretical limits for free rolling, a continuous additional force must be exerted on the product, even after the roll gap is filled. This condition is referred to hereinafter as "forced rolling".
In the past, the rolling schedules of continuous mills have conventionally operated under conditions of spontaneous entry and free rolling. Absent equipment failures or other unusual circumstances, this approach provides for a smooth passage of the product from one roll pass to the next, which of course is an essential requirement for successful mill operation.
However, it is also known that in any given roll pass, the reduction taken is directly proportional to the magnitude of the cosine of the bite angle. Thus, it will be appreciated that in conventional mills, by limiting the size of the bite angles to accommodate spontaneous entry, considerably less than maximum reductions are taken once the roll gaps are filled. If less than the maximum reductions are taken at the roll passes, their number must be increased in order to achieve a given total reduction.
The additonal roll passes and their associated drives, controls, lubricating and water cooling systems, etc. are extremely costly. The additional roll passes also contribute significantly to mill operating and maintenance costs, while occupying more building space, which is itself a high cost factor in any given mill installation. This latter expense is compounded in many mills by the provision of substantial interstand spacing.
As the costs of rolling equipment, buildings, energy, etc. continue to increase, there is a growing demand for more efficient high reduction rolling methods employing compact smaller sized equipment.
The idea of achieving higher reductions in the roll passes of rolling mills is not in itself new, and over the years those skilled in the art have advanced several proposals for doing so, including for example continuously forcing products through roll passes defined by undriven work rolls (U.S. Pat. No. 723,834) as well as through roll passes defined by driven work rolls (U.S. Pat. No. 4,106,318). However, a problem with these proposals is that they entail the use of relatively large diameter work rolls, which in turn require massive bearings, housings, mill foundations, etc., and large mill buildings. Thus any benefits derived from achieving higher reductions are largely offset by higher capital costs.
In another proposal disclosed in U.S. Pat. No. 3,553,997, high reductions are sought by employing relatively small diameter driven work rolls. Here, however, the roll gaps are initially opened to freely accept each front end, after which the roll gaps are closed to roll the remainder of the product. The impracticability of constantly opening and closing roll gaps, and the waste resulting from the scrapping of unrolled front ends, makes this method inapplicable to modern high tonnage rolling operations.
Other proposals for achieving high reductions include swing forges and planetary mills. While these approaches have met with some limited success in specialized low tonnage applications, they have not achieved widespread acceptance by the rolling mill industry.